1. Field of the Invention
The invention relates to a multi-beamlet charged particle lithography system and a method for transferring a pattern to a surface of a substrate, and more particularly relates to a lithography system having an alignment sensor and beam measurement sensor and a method of using the system.
2. Description of the Related Art
In the semiconductor industry, an ever increasing desire exists to manufacture smaller structures with high accuracy and reliability. Charged particle lithography is a promising technology to meet the high demands. In this type of lithography charged particles are manipulated to transfer a onto a target surface of a substrate, typically a wafer.
Lithographic processing generally involves exposure of multiple layers on top of each other so that features formed in subsequent layers may be connected so as to create an integrated circuit. Therefore a pattern to be exposed in a later exposure session is to be aligned with the one or more patterns that were created earlier in a sufficiently accurate manner. A position difference between a pattern in one layer and a pattern in a previously patterned layer on the same substrate is referred to as overlay.
To control overlay in a multi-beam charged particle exposure apparatus, such as a lithography apparatus, the relationship between the substrate to be exposed and the charged particle radiation is determined. For this purpose alignment marks are present on substrates to be processed. However, it is not possible to determine the relationship between the substrate to be processed and the charged particle radiation, or the projection system used to project such radiation onto the substrate in a direct way. Consequently, multiple measurements are needed which take valuable time.
A sensor for measuring properties of a large number of charged-particle beamlets, in particular for charged particle beamlets used in a lithography system, is described in US published patent application 2007/057204 assigned to the present applicant, the content of which is herewith incorporated by reference in its entirety.
US 2007/057204 describes a sensor and method in which charged-particle beamlets are converted into light beams, using a converter element such as a fluorescent screen or a doped YAG material. Subsequently, the light beams are detected by an array of light sensitive detectors such as diodes, CCD or CMOS devices. A relatively fast measurement can be achieved by reading out a large number of light sensitive detectors in a single operation. Additionally the sensor structure, in particular the array of light detectors, enables a very small pitch of a multiplicity of beams to be measured without the necessity of unduly large structural measures in the region of the stage part of a lithography system.
However, in view of the continuously increasing demands of the industry regarding small dimensions without loss of throughput, there remains a need to provide even more accurate devices and techniques for measurement of beamlet properties in lithography systems, particularly in lithography machines comprising a large number of charged-particle beamlets that are designed to offer a high throughput.